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Ilkka Seppälä 2014-08-24 23:17:26 +03:00
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##Observer
**Intent:** Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
![alt text](https://github.com/iluwatar/java-design-patterns/blob/master/observer/etc/observer.jpg "Observer")
**Applicability:** Use the Observer pattern in any of the following situations
* when an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
* when a change to one object requires changing others, and you don't know how many objects need to be changed
* when an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
##State
**Intent:** Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.